Not sure how it works with OSX, but write protection can be done on the file system level in Windows via NTFS. There's also software out there that will write protect a drive if you have to use another filesystem.

Would a (not micro) SD card + reader combo work for you? You could just write whatever you need and then use the little lock switch on the SD card.
I've booted off them in the past - just have to make sure the computer sees the reader as a USB drive and not some special card, e.g. Linux assigns it a /dev/sdX rather than /dev/mmcblkX.

__________________"The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src]

Not sure how it works with OSX, but write protection can be done on the file system level in Windows via NTFS. There's also software out there that will write protect a drive if you have to use another filesystem.

software level won't help me in this case. I'm not trying to lock files, I need to lock the hole drive into read-only at a hardware level so that if I boot from a live OS whether WinPE or linux, etc, I can't wipe the partition on the drive

A USB card ready could work, but it seems like a backwards way of doing things. I dunno, I'm really picky when it comes to getting what I want. It's 2013, we have the technology, but it seems there's always a compromise

and as mentioned, finding newer, large capacity keys with hardware locks is near impossible, finding a drive with encryption is no problem on the other hand, but I don't want encryption for a multiboot key

AKG: "So please don't piss on people and claim its raining. "
Soullessone21: "Their the only fat bitches I ever want in my wife :)"
DeeraxTheCoolNemo: "I never preSs CaPLoCK I just HiT my keyS so Hard ThEY Crit"

it's not just for OS installed, I have lots of diag and other troubleshooting tools in there, MS's recovery tools for different versions of windows in both x86 and x64, plus my own custom PE environment, again, both x86 and x64

and the OS install files includes all editions of XP, Vista, 7,8, server 2008, 2008 r2, and server 2012, they take up a lot of space

I don't get why you don't just label it physically and logically so that you know what drive it is and don't format it.

__________________
Random quotes:

AKG: "So please don't piss on people and claim its raining. "
Soullessone21: "Their the only fat bitches I ever want in my wife :)"
DeeraxTheCoolNemo: "I never preSs CaPLoCK I just HiT my keyS so Hard ThEY Crit"

its because i have some batch scripted OS installers in for windows in my custom winpe. the reason it's setup that way is because i have multiple editions of windows and versions of windows. i could have copied individual setup folders, but by using batch files, diskpart scripts and dism to apply unattended configurations, i was able to bypass the setup process, and combine the wim images to reduce the amount of diskpage.

so for example, windows 7 and server 2008 are combined into a single wim, same with windows 8 and server 2012. combining the rims cut down the disk cost by about 9gb

now because i did it this way, if there's no driver loaded to recognize the hard drive in the machine, when the diskpart script runs and cleans disk 0, well oops, there goes my usb

if my coding skills weren't limit to batch files, i would have happily made something better, that can check if there's an actual hard drive, but i'm no code junky.

so the next best thing is to get a drive with a write protect switch.

so far it seems the Kanguru SS3 is the only one i can find, and it's a little rich for my blood at 120$ almost for a 64gb